Stay at a hotel in Tokyo, and you will likely find several complimentary bottles of chilled spring water waiting in your room. As tempting as these may be to a road-weary traveler, guests before cracking one open should consider that some of the best drinking water in the city is a mere turn of the faucet away.

There is no shortage, however, of people who shun the idea of drinking from the tap and are instead willing to pay for pricy bottles of water. Experts are quick to point out, though, that the water sitting on store shelves and in vending machines is not necessarily of higher quality than that flowing from spigots, nozzles, and drinking fountains around Tokyo.

Setting the Bar High

It may seem strange that a sprawling metropolis such as Tokyo would offer some of Japan’s finest drinking water. But before a single drop reaches consumers it must clear 51 strict quality standards set by the Ministry of Health, Labor, and Welfare, including checks for toxicity and harmful contaminants, along with tests to ensure a palatable tint, clarity, and smell. In fact, Japan’s regulations for public water supplies are more stringent than those governing bottled spring water. What takes Tokyo’s water a step higher, though, is the metropolitan waterworks’ strict treatment regimen that includes roughly 200 parameters for safety and quality.

Hashimoto Takashi, an employee at the Tokyo Metropolitan Government Bureau of Waterworks, vouches for the water the utility supplies to the capital’s 13 million residents. Hashimoto, who heads the Misono Treatment Plant in Itabashi, confidently says that he no longer bothers with bottled water. “Why should I when safe, clean water flows from the tap.”

The Misono facility is one of five treatment plants run by the Tokyo Waterworks. It has a daily treatment capacity of 300,000 cubic meters and utilizes standard procedures along with advanced treatment of ozone and biological activated carbon. According to Hashimoto the advanced system removes nearly all the dissolved organic matter that standard treatment systems are unable to catch.

Leading-Edge Water Treatment

At the Misono plant, a window in the ozone contact chamber offers a glimpse at the interior of the treatment system. A 10-centimeter wide tube at the bottom of the tank diffuses a steady stream of minute bubbles that slowly ascend through the jade-tinted water. Ozone is a strong oxidizing agent and the bubbles in their effervescent journey to the surface react with inorganic materials, including trihalomethanes and other known carcinogenic compounds along with microorganisms such as bacteria and protozoa, eliminating potential health risks and improving the taste and smell of the water through nonchemical means.

Hashimoto says the ozone purification process takes around 20 minutes, pointing out that the miniscule size of the bubbles is a key factor in the treatment. “Fine bubbles improve efficiency by ensuring more ozone dissolves into the water and remains there over a longer period of time.” He points out, however, that the gas’s corrosive properties present some challenges for the plant. “It will quickly eat through a standard steel pipe, so we have to use stainless steel for the treatment facility.”

The ozone generator (left) is large enough for an adult to stand upright in, and (right) a window showing the inner workings of the ozone chamber.

In the second step of the advanced treatment system, water flows from the ozone chamber to filtration ponds containing different grades of biological activated carbon. In addition to filtering out organic contaminants, much like a home water purifier, the microporous carbon also supports microorganisms that break down impurities and other byproducts of ozone treatment. If left behind, these contaminants can degrade the taste and smell of water.

According to Ogasahara Atsushi, who oversees filtration at the Misono facility, the plant lays 2.5-meter thick beds of activated carbon in 100-square-meter filtration pools at the site. Ogasahara explains that active carbon is very effective at purifying water, but that the beds gradually loses the ability to absorb organic material and must be replaced every four years to maintain efficiency.

A window in a filtration pond shows the bottom layer of biological activated carbon.

The quality of source water determines the level of water treatment. Tokyo currently gets 80% of its water from the Tonegawa and Arakawa Rivers, drawing the remaining 20% from the Tamagawa River. Hashimoto explains that water from Tamagawa is pure enough that it only needs standard treatment to make it drinkable. The quality of raw water emanating from the first two rivers, however, is somewhat lower. But by applying advanced water treatment, the waterworks is able to bring the quality to an exceptionally high level.

(Clockwise from upper left) Sloped sedimentation basins efficiently remove suspended solids from raw water. Solar panels cover sand filtration ponds that are the last step in advanced water treatment. Settling basins and other facilities for industrial water treatment at the Misono plant.

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